1. Field of the Invention
The present invention relates to an optical information processing apparatus which can record information on an optical recording medium, reproduce the information recorded on the optical recording medium, and/or erase the information recorded on the optical recording medium, and more particularly, it relates to a focusing and tracking control circuit thereof.
2. Related Background Art
In the past, as a recording medium on which information is recorded by the use of light and from which recorded information is read-out, various kinds of recording mediums such as disk-shaped, card-shaped, tape-shaped, and the like, have been proposed.
For example, in an optical disk, information is recorded as optically detectable rows of bits (information tracks) by scanning the disk by use of a light beam modulated according to information to be recorded and stopped down to form a minute spot. In this case, in order to correctly record the information without difficulty, such as crossing of the information tracks, it is necessary to control a position on which the light spot is illuminated in a direction perpendicular to a scanning direction in a plane of the optical disk (auto-tracking, referred to a "AT" hereinafter). Further, in order to illuminate the light spot onto the optical disk as a minute spot having stable dimension, regardless of the curvature of the optical disk and/or mechanical errors, it is necessary to control the position of the light spot in a direction perpendicular to the plane of the optical disk (auto-focusing referred to as "AF" hereinafter). In addition, in the reproducing operation, the above-mentioned AT and AF must be considered. While there have been many kinds of such AT and AF techniques proposed in the past, in general, the one that has been utilized is a method in which an objective lens is selectively driven by respective AT and AF actuators in response to a focusing error signal and a tracking error signal on the basis of differences in light intensity in a plurality of light receiving surfaces of focusing and/or tracking photo-detectors which receive the reflected light or transmitted light by the recording medium.
However, in such AT and AF controls, a problem arisen in that malfunction has occurred in a servo system due to variation in the magnitude of such error signals in response to such as, for example, a change in intensity of the light beam illuminated onto the recording medium, or a change in reflection index of the recording medium. In order to solve such a problem, an optical information processing apparatus having a so-called automatic gain control (AGC) circuit for adjusting gain of a focusing control circuit in response to the reflection index of the recording medium has been proposed, as disclosed in the U.S. Pat. No. 4,701,603 and the like. An example of such an AGC circuit is shown in FIG. 1.
In FIG. 1, a reference numeral 1 designates a focusing error signal; 2 designates an AGC circuit for normalizing the focusing error signal and the tracking error signal; 3 designates a tracking error signal; and 4 designates a summation signal of outputs from the aforementioned plurality of light receiving surfaces of the photo-detectors. Here, the AGC circuit 2 can operate in such a manner that, when the summation signal is increased (i.e., the quantity of light from the medium is increased), the gains of the focusing control circuit and/or the tracking control circuit are decreased.
However, in the aforementioned prior optical information processing apparatus, for example, in a tracking seeking condition wherein the AT is operated and the AF is not operated, since the summation signal is modulated each time the track is crossed, a problem arises in that the error signals are not properly normalized, thus presenting an unstable factor in the pull-in of the tracking servo.